The invention relates to a method for determining the position on the upper side of the tibia of the exit point of an implant replacing the anterior cruciate ligament of a knee joint. The invention also relates to an apparatus for determining the position on the upper side of the tibia of the exit point of an implant replacing the anterior cruciate ligament of a knee joint, having a navigation system, having marking elements fixedly connected to the femur and the tibia, having a navigated sensor and having a data-processing system to which the positional data recorded by the navigation system can be fed.
When replacing an anterior cruciate ligament of a knee joint with an implant, it is extremely important for the implant to be positioned exactly. The implant is usually connected in a suitable way at its two ends to the femur and the tibia, respectively, for example by insertion in holes which are drilled in the femur and the tibia and the end points of which mark exit points for the implant on the surface of the femur and the surface of the tibia.
An important aspect here is that, when there is movement of the knee, the length of the implant is to remain as invariant as possible in the region between the exit points, in other words an isometry of the implant is to be achieved. For this purpose, it is known to determine the exit point of the implant on the femur by a method and an apparatus with which geometrical data of the femur surface are scanned and stored and with which these geometrical data are used to calculate the change in the isometry for different positions of the exit points (EP 603 089 B1). With this method, however, it is only possible to optimize the position of the exit point on the femur side; this known method cannot help with the selection of the exit point of the implant on the tibia side.
A major problem when determining the position of the exit point on the tibia side is that, when it is in the fitted state, the implant must remain free between the two exit points in all positions of the knee; it must therefore not be in contact with bone structures, for example specific surfaces of the femur, since this contact could lead to the implant being damaged. The anterior cruciate ligament and the implant replacing it are normally located in the interspace between the surfaces of the femur joint, that is the condyles, this interspace being referred to as the notch. This notch is bounded laterally by the side surfaces of the condyles, on the upper side by an upper roof area, and this roof area goes over into the lateral outer surface of the femur via a prominent front edge on the front side of the femur. If the implant of the anterior cruciate ligament is improperly positioned, it is precisely this front edge that can collide with this implant in certain positions of the leg, and this must be avoided in all cases.
Up until now, the surgeon has chosen the positioning of the tibial exit point of the anterior cruciate ligament on the basis of his experience, but misplaced implantations cannot in this case be ruled out with certainty.
It is an object of the invention to provide a method by which the surgeon is assisted in locating the tibial exit point in such a way that a free path for the implant is ensured in all positions of the leg.